CaesarCipher

CaesarCipher is a small Microsoft Foundry prompt-agent sample that uses an Azure Function app as a remote MCP tool.

The Foundry agent receives the user's request, calls the MCP tool exposed by the Function app, and returns the tool result. The Caesar transformation is owned by the Function app, not by the prompt agent.

[!WARNING] This is an experimental learning project and should not be considered production-ready.

What It Does

The agent handles requests such as:

• "Please encode the message: Hello"
• "Encode the following plaintext: Attack at dawn"

The prompt in prompt.md instructs the Foundry agent to call the MCP tool named caesar-cipher-function. The tool accepts a single plaintext argument and returns the encoded text.

Architecture

Foundry prompt agent
  -> remote MCP server connection
  -> Azure Functions MCP webhook
  -> caesar-cipher-function
  -> src/azure_functions/caesar_ciper.py

The Azure Functions MCP endpoint is:

https://{function-app-name}.azurewebsites.net/runtime/webhooks/mcp

Do not configure Foundry with only the Function App base URL. The base URL returns a normal web response, not MCP JSON-RPC messages.

Foundry Setup

Create and configure the agent in Microsoft Foundry:

1. Create or open a Foundry project.
2. Deploy a chat model in that project.
3. Create a prompt agent named caesar-cipher.
4. Paste the contents of prompt.md into the agent instructions.
5. Add a remote MCP server tool connection.
6. Set the remote MCP server endpoint to https://{function-app-name}.azurewebsites.net/runtime/webhooks/mcp.
7. Set authentication to unauthenticated for early testing.
8. Configure the allowed tool as caesar-cipher-function.
9. Save and test the agent in Foundry Chat.

The tool call should appear in Foundry Chat as a call to caesar-cipher-function with a plaintext argument.

Azure Function App

The deployable Azure Function app files live in src/azure_functions/.

For zip deployment, zip the contents of this directory so these files are at the zip root:

function_app.py
caesar_ciper.py
host.json
requirements.txt

function_app.py uses the Azure Functions Python v2 programming model and the @app.mcp_tool_trigger decorator to expose caesar-cipher-function.

host.json configures the Azure Functions MCP extension and currently allows anonymous access to the MCP webhook for testing. Add authentication before using this pattern outside a learning environment.

Local MCP Smoke Test

After deploying the Function app, test the MCP server directly before testing through Foundry:

.\scripts\test-mcp-endpoint.ps1 -Endpoint "https://{function-app-name}.azurewebsites.net/runtime/webhooks/mcp" -Plaintext "Hello"

For the current sample deployment, the script defaults to the known MCP endpoint, so this is enough:

.\scripts\test-mcp-endpoint.ps1 -Plaintext "Hello"

A healthy endpoint lists caesar-cipher-function from tools/list and returns the encoded text from tools/call.

Local Setup

Create the virtual environment and install the project with development dependencies:

.\scripts\setup-dev.ps1

The setup script expects Python 3.11 at the path configured in scripts/setup-dev.ps1.

Foundry CLI

The repository also includes a small local CLI for calling the Foundry prompt agent after the agent is available:

.\.venv\Scripts\python.exe -m caesar_cipher "Please encode the message: Hello"

The command calls the live Foundry endpoint and may incur Azure or model usage costs.

The current local CLI is configured for:

agent name: caesar-cipher
agent version: 1

If the project endpoint, agent name, or agent version changes, update src/caesar_cipher/cli.py.

Development Checks

Run formatting, linting, type checking, and tests:

.\scripts\check.ps1

This runs:

• ruff format .
• ruff check .
• pyright
• pytest

Project Structure

src/azure_functions/
  function_app.py    Azure Functions MCP tool trigger
  caesar_ciper.py    Caesar transformation implementation
  host.json          Azure Functions host and MCP extension settings
  requirements.txt   Function app deployment dependencies

src/caesar_cipher/
  __main__.py        Package entry point for python -m caesar_cipher
  cli.py             Foundry client setup and command-line entry point

scripts/
  setup-dev.ps1
  check.ps1
  test-mcp-endpoint.ps1
  test_mcp_endpoint.py

tests/
  test_caesar_ciper.py
  test_function_app.py
  test_smoke.py

prompt.md           Prompt instructions pasted into the Foundry agent

Notes

This project is a Foundry learning exercise, not a production cryptography system. A Caesar cipher is a historical substitution cipher and is not secure for protecting sensitive information.

Do not include secrets or sensitive personal data in test prompts. The local CLI calls the remote Foundry agent and sends prompts to the configured Foundry project.

Third-Party Notices

This project has direct runtime dependencies on third-party Python packages, including azure-ai-projects, azure-functions, azure-identity, and httpx. See each package's PyPI license metadata for full license and notice terms.

License

GNU General Public License v3.0. See the LICENSE file for details.